1. Field of the Invention
The present invention relates to thin film type insulated gate semiconductor devices formed on various types of insulating substrates, for example, glass substrates, silicon wafers having an insulating film formed thereon. More specifically, the present invention relates to thin film transistors (TFTs) or thin film diodes (TFDs) and integrated circuits using these thin film devices, for example, active matrix electro-optical devices such as liquid crystal devices. The present invention further relates to a manufacturing method of these devices. In particular, the present invention relates to a low temperature process in which the highest process temperature is preferably not higher than 700.degree. C., more preferably 650.degree. C. or lower.
2. Description of the Prior Art
Semiconductor devices which have TFT on an insulating substrate such as a glass, such as active liquid crystal display devices and image sensors in which TFTs are used to drive picture elements for example, have been developed in recent years. Glass substrates which have a strain point of not more than 750.degree. C., and typically of 550-680.degree. C., are generally used for these substrates in view of both mass productivity and cost. Hence, the highest process temperature when such glass substrates are used must be not more than 700.degree. C.
Thin film-like silicon semiconductors have generally been used for the TFTs. The thin film silicon semiconductors can be broadly classified into two types, namely those consisting of an amorphous silicon semiconductor (a-Si) and those consisting of a silicon semiconductor which has crystallinity. The amorphous silicon semiconductors can be manufactured comparatively easily through a vapor phase method with a low production temperature, and they are suitable for mass production. Therefore, the amorphous semiconductors are used most generally, but their properties, such as their electric field effect mobility and electrical conductivity for example, are poor when compared with those of silicon semiconductors which have crystallinity. Therefore, there is a considerable demand for the establishment of a method for the manufacture of TFTs using silicon semiconductors which have crystallinity for attaining high speed characteristics.
The characteristics of the gate insulating film are not a serious problem in the case of a TFT where amorphous silicon which has a small mobility has been used. For example, a silicon nitride film which has poor electrical characteristics when compared with silicon oxide can be used for the gate insulating film of a TFT in which amorphous silicon has been used. However, with a TFT in which a crystalline silicon film which has a high mobility is used, the characteristics of gate insulating films are very important as well as the characteristics of silicon films.
The demand for good quality gate insulating films has become very great, especially in view of the improvement in the technology for obtaining crystalline silicon films. In this connection, with a TFT having a crystalline silicon film in which the channel forming region is comprised of essentially one single crystal or a plurality of crystals and the orientations of all of the crystals are the same (such crystalline forms are known as a mono-domain), the existence of the grain boundaries hardly affects the characteristics of the device, unlike with the usual TFT in which the polycrystalline silicon is used, and the electrical characteristics are determined almost entirely by the characteristics of the gate insulating film.
More specifically, the crystal orientations of two crystals which form a grain boundary are different from one another in the usual polycrystalline structure and, as a result, a high grain boundary barrier is produced. However, even though it is comprised of a plurality of crystals, in a mono-domain structure the crystal orientations of the two crystals which form a grain boundary corresponding to a grain boundary in the usual polycrystalline material are the same and so the barrier at such a boundary is very low, and it is no different from a single crystal. Consequently, in a mono-domain structure the contribution of the grain boundaries to the TFT characteristics is very small, and the characteristics are determined mostly by the gate insulating film.
Thermal oxide films are known as excellent gate insulating films which are ideal for such a purpose. For example, gate insulating films can be obtained using the thermal oxidation method provided that they are on a substrate such as a quartz substrate which can withstand high temperatures. (For example, JP-B-H3-71793) (The term "JP-B" as used herein signifies an examined Japanese patent publication").
Thermal oxide films have very few defects which act as traps when charges such as hot electrons, for example, are implanted and so there is little deterioration in their characteristics, and it has been possible to produce elements which have a high degree of reliability.
A high temperature of at least 950.degree. C. is required to obtain a silicon oxide film which can be used as a gate insulating film by means of the thermal oxidation method, and there is no other substrate material apart from quartz which can withstand such high temperatures. A maximum process temperature of not more than 700.degree. C., and preferably of not more than 650.degree. C., is required if glass substrates which have a low strain point as described above are to be used, and it is impossible to satisfy this requirement with the thermal oxidation method.
Gate insulating films must be produced using physical gas phase growth (PVD) methods, such as the sputter method, or chemical gas phase growth (CVD) methods, such as the plasma CVD method and the thermal CVD method, because of these problems. A highest process temperature of not more than 650.degree. C. is a possibility with these methods.
However, insulating films which have been produced using the PVD methods and CVD methods have high concentrations of dangling bonds and hydrogen, and the boundary characteristics are not good. Consequently, they are weak in respect of the implantation of hot electrons, for example, and charge trapping centers are liable to be formed because of the presence of the dangling bonds and hydrogen. Consequently, when used as gate insulating films for TFT, there is a problem in that the electric field mobility and the sub-threshold characteristic value (S value) are not good, or there is a problem in that the leakage current of the gate electrode is considerable and the fall in the ON current (deterioration, change with the passage of time) is considerable.